This application relates to optical inspection equipment used to evaluate parameters of thin films on semiconductor wafers. The subject invention includes a cleaning module for reducing contaminants on the surface of the wafer prior to measurement to improve the accuracy and repeatability of the optical measurements. In the preferred embodiment, the cleaning module includes separate heating and cooling chambers for processing the wafer prior to measurement in the metrology tool.
For many years, devices have existed for evaluating parameters of a semiconductor wafer at various stages during fabrication. There is a strong need in the industry to evaluate the parameters of multiple-layer thin film stacks on wafers using non-contact optical metrology tools. In these devices, a probe beam of radiation is directed to reflect off the sample and changes in the reflected probe beam are monitored to evaluate the sample.
One class of prior measurement devices relied on optical interference effects created between the layers on the sample or the layer and the substrate. In these devices, changes in intensity of the reflected probe beam caused by these interference effects are monitored to evaluate the sample. In many applications, the probe beam is generated by a broad band light source and such devices are generally known as spectrophotometers.
In another class of instruments, the change in polarization state of the reflected probe beam is monitored. These devices are known as ellipsometers.
As thin films and thin film stacks have become more numerous and complex, the industry has begun developing composite measurement tools that have multiple measurement modules within a single device. One such tool is offered by the assignee herein under the name Opti-Probe 5240. This device includes a number of measurement modules including a broad band spectrophotometer and a single wavelength, off-axis ellipsometer. The device also includes a broadband rotating compensator ellipsometer as well as a pair of simultaneous multiple angle of incidence measurement modules. The overall structure of this device is described in PCT WO 99/02970, published Jan. 21, 1999, incorporated herein by reference. The Opti-Probe device is capable of deriving information about ultra-thin films and thin film stacks with a high degree of precision.
There is a trend in the semiconductor industry to utilize very thin layers. For example, today, gate dielectrics can have a thickness less than 20 xc3x85. It is anticipated that even thinner layers will be used. There is a need to measure the thickness of these very thin layers with a precision and repeatability to better than 0.1 xc3x85. While the Opti-Probe device is capable of making such measurements with the necessary precision, problems have arisen with respect to repeatability, especially with ultra thin films. Repeatability means that if the same measurement is made at two different times, the same result for layer thickness will be produced.
After considerable investigation, it has been determined that variations in measurements over time are strongly affected by atmospheric conditions such as temperature, humidity and exposure time to the air. For example, the measured layer thickness could be considerably higher when the humidity is relatively high. In addition, the thickness of the layer can be effected by the growth of a contaminant layer, even in so called xe2x80x9cclean roomxe2x80x9d environments. In fact, it is known that a clean room can contain a wide variety of contaminants including plastics, lubricants, solvents, etc. The variation in measurement due solely to atmospheric conditions can be on the order of 0.1 xc3x85 which substantially reduces the likelihood of making repeatable measurements with a precision of 0.1 xc3x85. In order to improve the repeatability of the measurements results, it would be desirable to remove the contaminant layer prior to measurement.
There are many types of wafer cleaning procedures used in a semiconductor fabrication facility. However, any cleaning procedures which require contact with the wafer, such as cleaning solutions, would not be desirable at this stage of fabrication since it can damage or contaminate the gate dielectric or the wafer. Additionally, most chemical cleaning processes require a drying cycle during which time a new hydrocarbon contamination layer could reform.
One suitable type of wafer cleaning system is described in our copending application Ser. No. 09/294,869, filed Apr. 20, 1999, and incorporated herein by reference. One embodiment of the system described in the latter patent application includes a microwave generator for exciting water molecules in order to drive off contaminants. Another approach described in the latter application was the use of a radiant heating source to drive off contaminants. Various additional combinations including microwave and radiant excitation along with UV radiation or a stream of frozen carbon dioxide pellets were suggested.
Another wafer cleaning system is described in PCT Application Ser. No. WO 99/35677 published Jul. 15, 1999. The device disclosed in this application relies primarily on radiant heating using tungsten halogen quartz lamps. An important aspect of the device in the latter application is the presence of high energy light wavelengths for breaking bonds in the contaminant layer. The wafer cleaner described in this PCT application has a single chamber. Cooling can be achieved through the use of a water-cooled bottom reflector in the chamber.
After considerable experimentation, it has been determined that the principal mechanism for removing contaminants in the approaches described above relates directly to an increase in the temperature of the wafer. Although microwave excitation and radiant light exposure both function to increase the temperature of the wafer, the latter two approaches are not the most efficient method of raising the temperature of the wafer. Therefore, it is believed that the best approach for preparing a wafer for measurement is to heat the wafer directly, by conduction.
Direct conductive heating has many advantages. For example, direct conductive heating can raise the temperature of the wafer to the desired temperature much faster than with either microwave or radiant energy exposure given the same amount of input energy. In addition, direct conductive heating can produce a more uniform and repeatable temperature rise in the wafer without complex equipment design.
Further experimentation also revealed that optimal results can only be achieved if the process is carefully controlled. Careful control includes heating each wafer to the same temperature, subjecting each wafer to the same cooling cycle and insuring that the time between the end of the cooling cycle and the beginning of the measurement cycle in the metrology tool is the same for all wafers.
Accordingly it is an object of the subject invention to provide an improved wafer-cleaning module which can accurately and repeatably remove contaminants from a wafer prior to measurement in a metrology tool.
A wafer-cleaning module is disclosed which includes a heating station having a planar heater element for heating the wafer by conduction. In the preferred embodiment, the heater element is a plate formed from a dielectric material such as alumina. The plate has a thin layer of a resistive material attached or deposited on the underside thereof. An electrical current applied to the resistive layer creates heat which diffuses evenly through the plate. A set of lift pins can be provided to raise and lower the wafer onto the plate. The lift pins are provided to permit a robotic arm to more easily load and remove the wafer from the heating station.
The wafer-cleaning module further includes a separate, thermally isolated cooling station. The cooling station includes a planar heat sink surface which can be air or water-cooled. Having separate heating and cooling stations allows the wafer to be cooled faster and more efficiently than if the cooling is performed within the heating station.
In accordance with the subject invention, the cleaning module is placed under the control of a processor. In order to achieve repeatability of the measurement of the wafer, the heating and cooling steps must be the same for each of the wafers being tested. To the extent possible, each wafer should be heated to roughly the same temperature and held at that temperature for roughly the same period of time. Each wafer should be subjected to the same cooling cycle. In addition, the time period between the end of the cooling cycle and the initiation of the measurement cycle should also be the same for each wafer.
In the preferred embodiment, a robotic arm loads the wafer into the heating station (chamber). The processor controls the heating chamber based on both time and feedback from temperature sensors in the chamber. When the heating cycle is complete, the robotic arm will transfer of the wafer from the heating chamber to the cooling station (chamber). After the cooling cycle is complete, the robotic arm will transfer the wafer to the metrology tool for measurement. As noted above, each of the various cycles and periods between cycles should be the same for each wafer.